Abstract:
“Ocean spice” refers to one of the great hidden mechanisms driving ocean missing. Large regions of the oceans are layered in such a way that differences in molecular characteristics between heat and the salt (technically called double diffusion) are sufficient to vigorously mix the seawater – but only in vertically confined layers. These layers form tendrils anywhere from 1 to 100 m thick and can ... extend for hundreds of kilometres. Yet we have few direct measurements of the rates of vertical mixing within and between these layers. It is likely that rates of mixing will differ by orders of magnitude within a few meters, or even mix in the opposite direction to “normal”. This process is seen in polar oceans where the interaction between seawater and melting ice (both sea ice and ice shelves) modulates ocean stratification and transport. In turn, stratification and transport strongly affect the combined climate-ecosystem. Here we will link field measurements of ocean spice generated by ice shelves with laboratory and computer models. The aim of the work is two-fold: better understanding of the details of this mixing and to fill in a key gap in predictive climate models.

Antarctic ice shelf cavities are one of the last unknown geophysical domains. The Ross Ice Shelf Cavity alone contains ~120,000 cubic km (1.3x that of the North Sea), of essentially un-sampled ocean. Geophysical-scale fluid mechanical modelling has the potential to identify the dominant energy and material pathways, but only if this modelling is strongly informed by the correct mechanics. At these scales oceanic transport and diffusion are dominated by mixing near sharp topography. Very recent ocean observations from the front of the Ross Ice Shelf suggest that this might also be true for ice shelf cavities.

Here we are using glacier tongues as an accessible proxy for the front of an ice shelf. We are looking at the downstream manifestation of this ice shelf water and how it is influenced by giant glacier tongues of which there are many along the Victoria Land Coast.

Oceanic mixing processes in the lee of a floating glacier tongue, the Erebus Ice Tongue, McMurdo Sound, were investigated to inform geophysical-scale fluid mechanical modelling of ice shelves. The Erebus Ice Tongue was used as an accessible proxy for the front of an ice shelf. Ocean flow rates and rates of mixing (= turbulence), temperature and salinity were measured next to the Erebus Ice Tongue and an AUV and ROV was deployed to make measurements at depth. Four different acoustic current profilers generated velocity data extending right from just beneath the ice-ocean interface to the seafloor bed (340m). Turbulence profile data (VMP 500 profiler) was obtained in 24 hour bursts (a profile every 20 minutes) during spring and neap tides.

The Drygalski Ice Tongue is the largest existing ice tongue remaining. Not only is it in the path of buoyant out flow from the Ross Ice Cavity it also heavily influences the Terra Nova Bay Polynya one of the key oceanographic features in the area. The main focus for the research was to determine flows beneath/next to glacier tongue, determine rates of ocean mixing and seek super cooling effects.